This invention relates to transducer housings which can be clamped to a surface into which sonic energy is to be exchanged with the housings, and more particularly relates to a metal transducer housing capable of being cast and which has a sonic lenticular mode conversion surface.
Transducer structures for flow monitoring equipment are known and are shown, for example, in U.S. Pat. Nos. 3,869,915, 3,987,674 and 4,373,401, all of which are in the name of Joseph Baumoel.
Application Ser. No. 407,434 referred to above discloses the advantages of a metallic transducer housing and the use of a mode conversion surface for converting longitudinal mode ultrasonic energy into shear mode ultrasonic energy which has a relatively low speed of propagation.
Metals such as zinc and brass are disclosed for the metallic housing. When using metals such as zinc and brass, it is necessary that the metals be processed in a manner that does not form large grains since a large grain structure interferes with the sonic conductivity of the metal. Thus, special treatment is needed and the metal cannot ordinarily be cast because the cast cooling process produces the undesired large grain structure. Therefore, the transducers must be machined to shape from rolled stock. The resulting transducer housing is expensive because of both the materials needed and their subsequent machining.
The problem of large grain size growth in metals such as brass, zinc or lead has been described in copending application Ser. No. 410,000, filed Aug. 20, 1982, entitled HIGHLY DAMPED TRANSDUCER STRUCTURE in connection with the manufacture of a damper having a sonically conductive interface with crystal. It is disclosed that in order to make this body sonically conductive, grain size must be reduced as by rolling. It is also disclosed that when the metals are in their .beta. state they contain numerous grain boundary dislocations which render the material unsuitable for a sonic conduction function.
A further problem in the prior art metal transducer structure is that the flat mode conversion surface tends to defocus the ultrasonic energy put out over the length of the so-called "footprint" or region from which ultrasonic energy leaves the transducer housing and enters the conduit. This defocusing is caused by the limited number of wavelengths in the footprint and causes aperture dispersion.
Another problem which has been experienced with metallic transducers is that transducer coupling fluids or oils cannot be effectively used for coupling a metal transducer housing to a metal conduit pipe wall and they are coupled only where the transducer and pipe wall are in actual contact. This is because the sonic impedance of the metal bodies is poorly matched to the sonic impedance of the coupling fluid. As a result, the only energy which enters the pipe wall from the housing is that energy appearing along the line of intersection of the flat metal housing output surface and the circular outer diameter of the metal pipe wall.
High Q transducer crystals had been used for the transducer housing in the prior art. These crystals, however, have numerous resonance modes which are difficult to match for a broad band type system.